Note - the above nat commands are commented out because even when clearing nat, nf_conntrack will be loadedwhich can cause issues.
See these links for more info:
http://www.pc-freak.net/blog/resolving-nf_conntrack-table-full-dropping-packet-flood-message-in-dmesg-linux-kernel-log/
http://antmeetspenguin.blogspot.com/2011/01/high-performance-linux-router.html